ASoC: qdsp6: q6apm-dai: unprepare stream if its already prepared

prepare callback can be called multiple times, so unprepare the stream
if its already prepared.

Without this DSP is not happy to setting the params on a already
prepared graph.

Fixes: 9b4fe0f1

 ("ASoC: qdsp6: audioreach: add q6apm-dai support")
Reported-by: Srinivasa Rao Mandadapu <quic_srivasam@quicinc.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20220610144818.511797-1-srinivas.kandagatla@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>